Generally, a check valve is installed in a fuel tank of a motor vehicle. The check valve prevents the rupture of the fuel tank by releasing fuel vapor from the fuel tank when the pressure thereinside increases to a certain value or higher, and prevents the collapse of the fuel tank by introducing outside air into the fuel tank when the pressure thereinside decreases to a certain value or lower than the atmospheric pressure.
As a conventional check valve, Patent Document 1 discloses a check valve including a first body, a second body, a positive-pressure valve and a negative-pressure valve. The second body is welded to the first body integrally therewith to form a valve chamber. The positive-pressure valve is disposed within the valve chamber, and the negative-pressure valve is slidably accommodated within the positive-pressure valve. The positive-pressure valve includes a valve body in which a first valve body and a second valve body are integrally welded and a first spring disposed between the valve body and the second body. A first valve having a communication hole in an apex portion thereof is formed at an upper portion of the first valve body, so that the first valve is brought into fitting contact with a first valve seat in the first body. The first valve body is biased towards the first valve seat by the first spring. The negative-pressure valve includes a second valve disposed slidably within the valve body and a second spring disposed between the second valve and the first valve body. The second valve is biased by the second spring so as be brought into fitting contact with a second valve seat at a lower portion of the second valve body. A first passage through which vapor flows out from a tank side to an external canister side is formed between an inner wall surface of the first body and an outer circumferential surface of the first valve body and a second passage through which air flows into the tank from the external canister side is formed between an outer circumferential surface of the second valve and an inner circumferential surface of the valve body.
When the pressure within the fuel tank increases to a certain value or higher, the positive-pressure valve is pressed by fuel vapor and slides against the biasing force of the first spring to open the first valve seat, whereby the fuel vapor flows through the first passage while passing across the first spring to thereby be discharged from the fuel tank. As a result, the pressure within the fuel tank decreases. On the other hand, when the pressure within the fuel tank decreases to a certain value or lower than the atmospheric pressure, the negative-pressure valve is pressed by outside air and slides against the biasing force of the second spring to open the second valve seat, whereby the outside air flows through the second passage while passing across the second spring to thereby be flown into the fuel tank through the valve chamber and the communication hole in the first valve. As a result, the pressure within the fuel tank increases.
Patent Document 1: JP-3421828-B